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Facies and Mafic Metamorphic Facies and Metamorphosed Mafic Rocks l V.M. Goldschmidt (1911, 1912a), contact Metamorphic Facies and metamorphosed pelitic, calcareous, and Metamorphosed Mafic Rocks psammitic hornfelses in the Oslo region l Relatively simple mineral assemblages Reading: Winter Chapter 25. (< 6 major minerals) in the inner zones of the aureoles around granitoid intrusives l Equilibrium mineral assemblage related to Xbulk Metamorphic Facies Metamorphic Facies l Pentii Eskola (1914, 1915) Orijärvi, S. l Certain mineral pairs (e.g. anorthite + hypersthene) Finland were consistently present in rocks of appropriate l Rocks with K-feldspar + cordierite at Oslo composition, whereas the compositionally contained the compositionally equivalent pair equivalent pair (diopside + andalusite) was not biotite + muscovite at Orijärvi l If two alternative assemblages are X-equivalent, l Eskola: difference must reflect differing we must be able to relate them by a reaction physical conditions l In this case the reaction is simple: l Finnish rocks (more hydrous and lower MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5 volume assemblage) equilibrated at lower En An Di Als temperatures and higher pressures than the Norwegian ones Metamorphic Facies Metamorphic Facies Oslo: Ksp + Cord l Eskola (1915) developed the concept of Orijärvi: Bi + Mu metamorphic facies: Reaction: “In any rock or metamorphic formation which has 2 KMg3AlSi 3O10(OH)2 + 6 KAl2AlSi 3O10(OH)2 + 15 SiO2 arrived at a chemical equilibrium through Bt Ms Qtz metamorphism at constant temperature and = 3 Mg Al Si O + 8 KAlSi O + 8 H O pressure conditions, the mineral composition is 2 4 5 18 3 8 2 controlled only by the chemical composition. We Crd Kfs are led to a general conception which the writer proposes to call metamorphic facies.” 1 Metamorphic Facies Metamorphic Facies Dual basis for the facies concept l Descriptive: relationship between the Xbulk & mineralogy l Interpretive: the range of temperature and pressure F A fundamental feature of Eskola’s concept conditions represented by each facies F A metamorphic facies is then a set of repeatedly F Eskola aware of the P-T implications and associated metamorphic mineral assemblages correctly deduced the relative temperatures and F If we find a specified assemblage (or better yet, pressures of facies he proposed a group of compatible assemblages covering a F Can now assign relatively accurate temperature range of compositions) in the field, then a certain and pressure limits to individual facies facies may be assigned to the area Metamorphic Facies Metamorphic Facies Eskola (1920) proposed 5 original facies: l In his final account, Eskola (1939) added: F Greenschist F Granulite F Amphibolite F Epidote-amphibolite F Hornfels F Glaucophane-schist (now called F Sanidinite Blueschist) F Eclogite ... and changed the name of the hornfels facies to the pyroxene hornfels facies l Easily defined on the basis of mineral assemblages that develop in mafic rocks Metamorphic Facies Metamorphic Facies Temperature Several additional facies types have been proposed. Most notable are: Sanadinite Formation of Zeolites Facies F Zeolite F Epidote- Pyroxene- Prehnite-pumpellyite Greenschist Amphibolite Amphibolite Hornfels Facies Facies Facies Facies ...resulting from the work of Coombs in the “burial metamorphic” terranes of New Zealand Pressure Granulite Fyfe et al. (1958) also proposed: Facies F Albite-epidote hornfels Glaucophane- Schist Facies Eclogite F Hornblende hornfels Facies Fig. 25-1 The metamorphic facies proposed by Eskola and their relative temperature- pressure relationships. After Eskola (1939) Die Entstehung der Gesteine . Julius Springer. Berlin. 2 Metamorphic Facies Metamorphic Facies l Table 25-1. The definitive mineral assemblages that characterize each facies (for mafic rocks). Fig. 25-2. Table 25-1 . Definitive Mineral Assemblages of Metamorphic Facies Temperature- pressure diagram showing the Facies Definitive Mineral Assemblage in Mafic Rocks generally accepted Zeolite zeolites: especially laumontite, wairakite, analcime limits of the various facies used in this Prehnite-Pumpellyite prehnite + pumpellyite (+ chlorite + albite) text. Boundaries are approximate and Greenschist chlorite + albite + epidote (or zoisite) + quartz ± actinolite gradational. The Amphibolite hornblende + plagioclase (oligoclase-andesine) ± garnet “typical” or average continental Granulite orthopyroxene (+ clinopyrixene + plagioclase ± garnet ± geotherm is from Brown and Mussett hornblende) (1993). Winter Blueschist glaucophane + lawsonite or epidote (+albite ± chlorite) (2001) An Introduction to Eclogite pyrope garnet + omphacitic pyroxene (± kyanite) Igneous and Metamorphic Contact Facies Mineral assemblages in mafic rocks of the facies of contact meta- Petrology. Prentice morphism do not differ substantially from that of the corresponding regional facies at higher pressure. Hall. After Spear (1993) Metamorphic Facies It is convenient to consider metamorphic facies in 4 2) Facies of medium pressure groups: 1) Facies of high pressure F Most metamorphic rocks now exposed belong to the greenschist, amphibolite, or granulite facies F The blueschist and eclogite facies: low molar F The greenschist and amphibolite facies conform to the volume phases under conditions of high pressure “typical” geothermal F Blueschist facies occurs in areas of low T/P gradient gradients, characteristically developed in subduction zones F Eclogites are stable under normal geothermal conditions May develop wherever mafic magmas solidify in the deep crust or mantle: crustal chambers or dikes, sub- crustal magmatic underplates, subducted crust that is redistributed into the mantle Metamorphic Facies Metamorphic Facies 3) Facies of low pressure 4) Facies of low grades F Albite-epidote hornfels, hornblende hornfels, and F Rocks often fail to recrystallize thoroughly at very low pyroxene hornfels facies: contact metamorphic grades, and equilibrium is not always attained terranes and regional terranes with very high geothermal gradient. F Zeolite and prehnite- pumpellyite facies are F Sanidinite facies is rare- limited to thus not always xenoliths in basic represented, and the magmas and the greenschist facies is innermost portions of the lowest grade some contact aureoles developed in many adjacent to hot basic regional terranes intrusives 3 Metamorphic Facies Combine the concepts of isograds, zones, and facies F Examples: “chlorite zone of the greenschist facies,” the “staurolite zone of the amphibolite facies,” or the “cordierite zone of the hornblende hornfels facies,” etc. F Metamorphic maps typically include isograds that define zones and ones that define facies boundaries F Determining a facies or zone is most reliably done when several rocks of varying composition and mineralogy are available Fig. 25-9. Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the medium P/T facies series. The approximate location of the pelitic zones of Barrovian metamorphism are included for comparison. Winter (2001) Facies Series A traverse up grade through a metamorphic terrane should follow one of several possible metamorphic field gradients (Fig. 21-1), and, if extensive enough, cross through a sequence of facies Figure 21-1. Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill. Facies Series l Miyashiro (1961) proposed five facies series, most of them named for a specific representative “type locality” The series were: 1. Contact Facies Series (very low -P) Fig. 25-3. Temperature- 2. Buchan or Abukuma Facies Series (low -P pressure diagram regional) showing the three major 3. Barrovian Facies Series (medium-P types of metamorphic regional) facies series proposed by 4. Sanbagawa Facies Series (high-P, Miyashiro (1973, 1994). Winter moderate-T) (2001) An Introduction to Igneous and 5. Franciscan Facies Series (high-P, low T) Metamorphic Petrology. Prentice Hall. 4 Metamorphism of Mafic Rocks Metamorphism of Mafic Rocks l Mineral changes and associations along T-P gradients Plagioclase: characteristic of the three facies series l More Ca-rich plagioclases become progressively F Hydration of original mafic minerals generally required unstable as T lowered F If water unavailable, mafic igneous rocks will remain largely l General correlation between temperature and unaffected, even as associated sediments are completely re- maximum An-content of the stable plagioclase equilibrated F At low metamorphic grades only albite (An0-3) is stable F Coarse-grained intrusives are the least permeable and likely to resist metamorphic changes F In the upper-greenschist facies oligoclase becomes stable. The An-content of plagioclase thus jumps from An1-7 to F Tuffs and graywackes are the most susceptible An17- 20 (across the peristerite solvus) as grade increases F Andesine and more calcic plagioclases are stable in the upper amphibolite and granulite facies l The excess Ca and Al ® calcite, an epidote mineral, sphene, or amphibole, etc., depending on P-T-X Metamorphism of Mafic Rocks Mafic Assemblages at Low Grades l Clinopyroxene breaks down to a number of mafic l Zeolite and prehnite-pumpellyite facies minerals, depending on grade. l Do not always occur - typically require unstable l These minerals include chlorite, actinolite, protolith hornblende, epidote, a metamorphic pyroxene, etc. l
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